KT-ORAM: An Efficient ORAM Built on k-ary Tree of PIR Nodes

This paper proposes KT-ORAM, a new hybrid ORAM-PIR construction, to protect a client’s access pattern to outsourced data. KT-ORAM organizes the server storage as a k-ary tree with each node acting as a fully-functional PIR storage, and adopts a novel delayed eviction technique to optimize the eviction process. KT-ORAM is proved to protect the data access pattern privacy at a failure probability negligible in N (N is the number of exported data blocks), when system parameter k = logN . Under the same configuration, KT-ORAM has an asymptotical communication cost of O( logN log logN · B) when the recursion level on meta data is of O(1) depth, which can be achieved if block size B = N ε (0 < ε < 1), or O( log 2 N log logN · B) when the number of recursion levels is O(logN). The costs of KT-ORAM are compared with those of several state-of-the-art ORAMs. The results show that, KT-ORAM achieves the best communication, storage and client-side computational efficiency simultaneously, at the price of requiring O( log 2 N log logN · B) computational cost at the storage server for each data query.